1. Field of the Invention
This invention relates to improvements in driver circuits and methods for voice coils of the type used to move read/write heads in a disk drive of the type used in computer systems, or the like, and more particularly to improvements in circuitry and methods for sensing currents in power driver transistors for a voice coils in a disk drive system, and still more particularly to improvements in circuits and methods for improving the accuracy of current sensing for voice coils in a disk drive systems and for positioning the heads controlled thereby.
2. Relevant Background
Typical disk drives, such as those used in floppy, hard and CD-ROM disk drives used personal computers, or the like, have one or more rotating disks onto which data can be selectively recorded and read. Typically, CD-ROMs have one or more rotating disks onto which data has been optically pre-recorded, which can only be read; however, recently, CD-ROM system have been introduced in which data can be written to an optical disk, which is nevertheless referred to as a "CD-ROM". In all such disk drives, however, one or more heads that record and read data, in the case of a magnetic disk drive, or merely detect data, in the case of a CD-ROM drive, are provided. In either case, the heads are typically carried on an arm assembly that is selectively positionable to any one of a number of circular data paths arranged at different radii of the disk from the spindle on which the disk is carried.
The arms on which the heads are carried, and, therefore the heads themselves, are moved along a line extending across the disk radii by a so-called "voice coil". By applying a current of known magnitude and polarity to the voice coil, the heads can be selectively positioned anywhere on the line relative to the disk.
In a typical system, the currents to the voice coil are controlled by a current driver circuit that includes four driver transistors. The driver transistors may be, for example, FET devices, connected to form an "H" bridge between a supply voltage, or V.sub.CC, and ground, with the voice coil being connected between the legs of the "H" and with each driver transistor connected in a respective upright of the "H". The FET devices connected to the supply voltage and the respective sides of the voice coil are generally referred to as the "high side drivers," and the FET device connected between the respective sides of the voice coil and ground are generally referred to as the "low side drivers." The respective high and low side drivers of each leg of the driver are generally controlled by the outputs of two oppositely driven power amplifiers.
The most common approach for controlling the movement of the heads is to use a sense resistor in series with the voice coil to sense the current flowing in the voice coil. The sensed currents are fed back to the driver amplifiers to enable the currents in the voice coil to be precisely controlled. Using sense FETs to sense the current in the coil reduces the resistive drop in the voice coil, which allows the customer to supply more current to the coil.
Thus, to detect the currents flowing in the power transistors in the current flow paths of the motor, FET devices may be connected as current mirrors to mirror the current in the power transistors. The mirror transistors are typically made small compared to the size of the power transistors, ratios of 15/1 being typical. It is, however, desirable to make the mirror transistors very small in comparison to the power transistors, but design tolerances result in greater imprecision amongst various mirroring transistors as the size ratio increases.
Nevertheless, the use of such FET mirror devices often produces poor accuracy because of the large size difference between the power devices and the mirror devices. Normally current flowing through one transistor can be mirrored to another transistor with relatively good accuracy if the devices are of similar size. In voice coil applications, however, in power FET devices, amps of current flow in the primary devices, and it is desirable to use a large ratio (on the order, for example, of 1500:1) between the power FET and the mirror device. If a smaller ratio were to be used, a large amount of power would be wasted in the control circuitry because of the large current in the mirror device. However, with a mirror ratio on the order of 1500:1, the accuracy if the mirror will not be very good, and can be expected to vary on lot and device basis.
In addition, the problem is complicated in that voice coil driving circuits may require multiple gain ranges, for example, three or more ranges, depending upon the particular system. These gain ranges allow the DAC that controls the voice coil loop to operate with equal accuracy over several ranges of coil current. Thus, if sense FET technology is used, as the feedback current becomes increasingly smaller, and potentially less accurate as the coil current approaches zero, many voice coil circuits increase the gain of the feedback current. In this manner, the current mirrors in the feedback block are not forced to handle currents from a few microamps to a few hundred microamps. This helps the matching of the current mirrors and, consequently the accuracy of the current control. Thus, in the past, the mirroring of the current in the driving transistors has been inaccurate, particularly at the low current levels that may be experienced in variable gain voice coil driving circuits.
What is needed, therefore, is a circuit and method that provide sensing current accuracy through mirror transistors of relatively small size, which may be used in conjunction, if desired, with circuitry to provide multiple gain ranges in which the feedback circuitry operates.